It is known that a nano uneven structure body having a nano uneven structure on its surface exhibits antireflective performance according to a change in a continuous refractive index. The nano uneven structure body can also exhibit super-water-repellent performance according to a lotus effect. However, the surface of the nano uneven structure has a nanoscale projecting part which is apt to be inclined, and has lower scratch resistance and durability than a smooth surface made of the same resin.
The following methods are proposed as a method for forming the nano uneven structure: for example, a method for performing injection molding and press molding using a stamper having a reversal structure of a nano uneven structure; a method for interposing an active energy beam-curable resin composition (hereinafter; referred to as a “resin composition”) between a stamper and a transparent substrate, irradiating the resin composition with an active energy beam to cure the resin composition, transferring the uneven shape of the stamper, and thereafter peeling off the stamper; and a method for transferring the uneven shape of a stamper to a resin composition, peeling off the stamper, and thereafter irradiating the resin composition with an active energy beam to cure the resin composition. Of these, the method for irradiating a resin composition with an active energy beam to cure the resin composition and transferring a nano uneven structure is suitable in light of transferability of the nano uneven structure and a degree of freedom of a surface composition. This method is particularly suitable when a belt-like or roll-like stamper capable of being continuously produced is used, and has excellent productivity. However, a resin having high cross-link density and a high elastic modulus is used in order to suppress the inclination of the projecting part during mold releasing of the stamper and by heating.
Intervals between the adjacent projecting parts or recessed parts need to be a size equal to or less than a wavelength of visible light in order for the nano uneven structure to exhibit good antireflective performance. The nano uneven structure body has lower scratch resistance than a molded product such as a hard coat produced using the same resin composition and having a smooth surface, and has a problem with durability in use. When the resin composition used for producing the nano uneven structure body is insufficiently robust, a phenomenon in which projections are disposed close to each other is apt to be generated by mold releasing from a mold and heating.
Heretofore, there are proposed a nano uneven structure body for which a nano uneven structure is formed using a method comprising irradiating a resin composition with an active energy beam to cure the resin composition and transferring the nano uneven structure, and a resin composition for forming a nano uneven structure. However, both involve cured products having high cross-link density and a high elastic modulus.
For example, Patent Literature 1 describes production of a nano uneven structure having a size equal to or less than the wavelength of visible light using closest packed silica sol as a mold. A multifunctional monomer having an extremely high double bond number per molecular weight such as trimethylolpropane triacrylate is used as a resin composition for forming the nano uneven structure.
Patent Literature 2 describes that a hard coat layer having fine unevenness is desirably a resin having hardness of “H” or greater in a pencil hardness test according to JIS K5600-5-4 (Paragraph 0022). Example thereof uses a multifunctional monomer having an extremely large number of double bonds per molecular weight such as dipentaerythritol hexaacrylate, dipentaerythritol pentaacrylate, and pentaerythritol tetraacrylate.
There are also reported an intermediate layer (Patent Literatures 2 and 3) that improves the adhesion and adhesiveness between a substrate film and a surface layer of a nano uneven structure, a laminate provided with a refractive index adjusting layer (Patent Literature 4) of a lower layer of a surface of a nano uneven structure in order to improve an antireflective effect, and an antireflective film (Patent Literature 5) including an intermediate layer having a function (self-restoring function) for restoring recessed flaws and a hard coat layer provided on the intermediate layer and having a different refractive index.
However, the nano uneven structure bodies described in Patent Literatures 1 to 4 do not necessarily satisfy scratch resistance. Even when the cured resin has hardness of “H” or greater in the pencil hardness test, the fine projection may be broken or bent to impair antireflective performance, particularly in the case of the nano uneven structure body, and the application thereof is limited. Even when the intermediate layer is provided, the intermediate layer is intended to improve adhesiveness and antireflective performance, and the scratch resistance thereof depends on the physical properties of the resin constituting the nano uneven structure. Although the antireflective film described in Patent Literature 5 has the intermediate layer having a self-restoring function to dent caused by pressing, the antireflective film may exhibit insufficient scratch resistance. For example, the thickness of the hard coat layer, the top layer, is as thin as 0.1 μm. When this is pushed in, the hard coat layer is broken, and is apt to be damaged in the pencil hardness test.